11.9.3 Private Functions

In many cases one function needs to access one or more helper
functions. If the helper function is limited to the scope of a single
function, then subfunctions as discussed above might be used. However,
if a single helper function is used by more than one function, then
this is no longer possible. In this case the helper functions might
be placed in a subdirectory, called "private", of the directory in which
the functions needing access to this helper function are found.

As a simple example, consider a function func1, that calls a helper
function func2 to do much of the work. For example:

function y = func1 (x)
y = func2 (x);
endfunction

Then if the path to func1 is <directory>/func1.m, and if
func2 is found in the directory <directory>/private/func2.m,
then func2 is only available for use of the functions, like
func1, that are found in <directory>.